Method for producing strontium phosphate



United States Patent Ofi ice Patented Dec. 10, less 3,113,335 Mnrnon iron Pnonocuso TRQNTIUM rnosrHATE Michael A. Ala, Towanda, 9a., assignor, by mesne assignmerits, to Sylt ania Electric Products inc, Wilmington, Del, a corporation of Delaware No Drawing. Filed luly 28, 1958,. Ser. No. 751,196

4 Claims. (Cl. 23109) This invention relates to new forms of strontium phosphate and to methods of preparation thereof. In particular, it relates to, and to the preparation of, two new forms of strontium hydrogen orthophosphate, denoted by the chemical formula SrHPO which is also called dibasic strontium phosphate, secondary strontium phosphate or distrontium phosphate. For convenience I will herein refer to the material as dibasic strontium phosphate.

1 have discovered that the compound exists in two separate and distinct crystalline forms, each of which gives specific X-ray diffraction patterns, dilferent from that of the other.

I have discovered that one form, which I have called the alpha form, designated by the Greek letter a, is precipitated by reactions in solution at temperature of 40 C. or higher, with the pH of the reacting solutions lying between about 3 and 5 until the equivalence point is approached. This form has small particle size, low specific B.E.T. (Brunauer, Emmett and Teller; Journal American Chemical Socity, vol. 60, page 309, year 1938) surface area, and low bulk density.

I have also discovered that another form, which I have called the beta form, designated by the Greek letter ,8, is precipitated at temperatures of 25 C. or below, with the pH of the reaction lying between about 5 and 7 until the equivalence point is approached. This form has larger particle size, much larger BET. surface area and higher bulk density.

Prior to my discovery, the commercially available dibasic strontium phosphate was composed of mixtures of the alpha and beta forms, although that fact was not known, because the existence of two forms had not been discovered. The percentages of the two forms in various commercial material, even Special Luminescent Grade, varied from manufacturer to manufacturer, and caused variations in the luminescent output of the fluorescent phosphate phosphors made from different batches of starting material.

By using the material of my invention, in which the phosphate is either substantially entirely of the alpha or beta type or of a mixture of known proportions of those types, the resultant phosphor will be reproducible from bat-ch to batch. To produce a phosphor, the dibasic strontium phosphate is generally mixed with activating compounds and the proper proportions of strontium carbonate to form the pyrophosphate, or whatever other phosphate is desired.

PREPARATlON OF a-SrHPO It is possible to utilize the simple reaction between (a) water soluble salts of strontium, such as chlorides, nitrates, acetates and the like, and ([2) water soluble monohydrogen phosphate salts such as those of the alkali metals and ammonia, the best reaction occurring with diammonium hydrogen phosphate, (NHQ HPO as indicated below:

The critical processing parameter is the temperature, which should be 40 C. or higher. The pH of the reaction, using SrC1 .6l-l;,O and (NHQ HPO should be approximately between 3 and 5 until the equivalence point is approached. The concentrations and rate of addition of (NHQ HPO are not especially critical in the formation of wSrl-lPO although these must be controlled since they effect ultimate physical properties of the precipitate. Addition of phosphate ion to that of strontium is desirable since the reverse procedure leads to colloidal precipitates which filter with great difficulty.

Calculations for a typical batch, based on a 100 lb. yield of SXHPO4, are as follows:

Sr(NOs)2 (NH4)2HPO4 SrHPO; 2NHiNOs The figures given represent the pounds of each material. The amount of (NH4)2I{PO4 includes 5% excess over stoi hiometric amount to insure completeness of precipitation. Concentrations up to about 2 molar are satisfactory for reactants. The (NHQ HPCL; solution may be added to the Sr(N0 solution or the two reactants may be added simultaneously to a third vessel under agitation. Note that 87.2 lbs of NH NO (in solution) is formed as a by-product of this precipitation. This material is easily removed by decantation. All but adsorbed nitrate ion can be removed by a series of water washes until a negative test is obtained using a suitable qualitative test on the wash water. After filtration, the :x-SrHPCL; may be dried at temperatures up to about 300 C. Batch yields are quantitative and assays are in the order of 98l00% of theoretical, as shown in the following Table I.

Table I SOME PROPERTIES OF a-SrHPOa PRECIPITATED Al VARIOUS TEMPERATURES BY THE ADDITION 013 1 M (NH4)2HPO4 TO 1 M SI'Cl2.6H2O

Pptn. Fisher Specific Bulk Wt. Percent Temp. SSS Surface Dena. SrHPO4 0.) (microns) Area, (gJinfl) Assay B.E.T. Sr P (percent) (m /a) Calculated for ErHPO 47. 7 16. 9

The effect of temperature on the physical properties considered is readily seen from Table I.

PREPARATION OF B-Srl-IPQ;

Table II 2. The method of preparing the beta form of dibasic strontium phosphate, said method comprising dissolving SOME PROPERTIES OF BSrHPOi PRECIPITATED AT VARIOUS TEMPER- ATURES BY THE ADDITION OF (NHMHPOi TO STRONTIUM SALTS C. but above about C., to precipitate out the dibasic strontium phosphate.

Pptn. Fisher B.E.'l. Bulk Wt. Percent Si'HPO Temp. Sr Salt SSS Surface Dens. Assay 0.) (microns) Area (g./in. (percent) (m. /g-) Sr P 0 sI'Ol2-6H20 13.0 19.6 10. 47.3 16.3 98.5 17 Sr(NO3)z 3.7 20.6 M" 25 SI(NO2)2 4.0 8.7 47.4 16.1 98.6

The higher B.E.T. area of [3-SrHPO compared to the strontium chloride in water and adding thereto ama-SIHPO of Table I is evident. Note that the analytical monium hydrogen phosphate to precipitate out the dibasic data (percent Sr and percent P) agree with those for 0astrontinum phosphate, the temperature of the solution S HPO being kept below about 25 C. but above about 0 C.

3. The method of prenaring the beta form of dibasic q Y "F Q g 8 S Or strontium phosphate, said method comprising dissolving in I 4. 5' 1 4 Water a water-soluble strontium salt, adding thereto a The process for mixed SrHPO powders is again, basic- Watebscluble monohyqrogen Phosphate Salt While p" n the same as that for wSrHPO4 The and ing the resultant solution at a temperature below about ,8-SrHPO structures are enantiotropic in the temperature but 2530)"? about and at a pH of between range about C. depending on the PH and the 25 about 5 to about until the equivalence point is apstrontium salt used. Precipitation at pH less than 5 and Proached, p p out dlbaslc Strontium P temperatures at the higher end of the 2050 C. range P I favor mixtures richer in oc-SIHPO Conversely, pH 1 T method of pnfparmg the beta d1ba $1c above 5 and lower temperatures favor mixtures richer in stwnqum P Q Sald method COIFIPTISmg dlssolvlng S HFQ, B dj m f H and temperature it is strontium chloride in Water and adding thereto ammopossible to obtain all co-precipitated mixtures from 100% mum hydrog5n phosphat3 to Preclpltate out (the filbaslc WSIHPO; to 100% fl SrHPO4 Suitable methods of esti strontium phosphate, the pH of the resultant solution bemating the amount of each form presant ing kept between about 5 to about 7 until the equivalence (a) X ray diffracfion analysis point is approached, the temperature of the solution being (b) Particle size distribution (by sedimentation). kept bfilow about but above about Various modifications will be apparent to those skilled in the art, without departing from the spirit and scope Reference Cited In the file of thls patent of the invention. UNITED STATES PATENTS What I clalm 1S1 2,366,270 Lemmers Jan. 2, 1945 T method of Pt p the beta. 0f 2,447,210 Roberts Aug 17, 194 strontium phosphate, said method comprising dissolving in water a water-soluble strontium salt, adding thereto a OTHER REFERENCES wfl'ief-solublfi THOHOhYdTQgeII Phosphate Salt While keeping Lexicon Hofirnan, Band 1, 1. Halfte, Wasserstoif Bis the resultant solution at a temperature below about 25 Sm N 1 31, 1917, page 419,

Grnelin-Ki'aut: Handbuch der Anorganisch Chemie, 22, 1909, page 170. 

1. THE METHOD OF PREPARING THE BETA FORM OF DIBASIC STRONTIUM PHOSPHATE, SAID METHOD COMPRISING DISSOLVING IN WATER A WATER-SOLUBLE STRONTIUM SALT, ADDING THERETO A WATER-SOLUBLE MONOHYDROGEN PHOSPHATE SALT WHILE KEEPING THE RESULTANT SOLUTION AT A TEMPERATURE BELOW ABOUT 25* C. BUT ABOVE ABOUT 0*C., TO PRECIPITATE OUT THE DIBASIC STRONTIUM PHOSPHATE. 